Semiconductor electric power switching circuits such as pulse width modulation (PWM) converter circuits and direct current (DC) to alternating current (AC) inverter circuits typically employ thyristors as switching devices. A preferred form of such thyristors is the gate turn-off (GTO) thyristor since it requires only a relatively simple drive circuit for gating the GTO device into and out of conduction. In some applications, such as in motor drive circuits in which the GTO device may carry several hundred amperes current and be required to operate with applied voltages of several hundred volts, it is desirable to monitor the status of the GTO device, including the temperature of the device, since excessive temperature may degrade the ability of the GTO device to function properly. In some systems, the actual gating signals applied between a gate and a cathode of GTO device are monitored to confirm their application to the device in response to signals from a control unit. The control unit is typically a microprocessor based controller which establishes the conduction and non-conduction time periods of the GTO device in a manner to regulate power to the apparatus, e.g., a motor, connected to the device.
In some prior systems, the monitoring function utilizes feedback status signals via a monitoring channel to confirm status of the switching devices. The status signals may be an indicator of the gate-to-cathode drive signals applied to each GTO device. Additionally, the system may include feedback signals indicative of the temperature of the GTO device. In the illustrative relatively high voltage motor drive system, temperature monitoring has been achieved by a heat sensitive element, such as a thermistor, coupled in heat exchange relationship with the GTO device or to a heat sink on which the GTO device is mounted. In either instance, the thermistor is electrically isolated from the GTO device voltage. Data obtained from the thermistor and from other monitored characteristics is transferred to the control unit through separate channels. In a system of six GTO devices, it is common to monitor each device along with each respective gate driver thus requiring twelve separate channels for data transmission. Each such channel requires separate wiring and its own electrical circuit hardware for supplying such data. It is therefore desirable to provide a method and apparatus for monitoring such GTO devices in a manner which reduces the wiring and hardware requirements.